British Patent 943,449 discloses a system for starting a gas turbine which utilizes compressed air stored in a reservoir for starting a combustion engine. After the combustion engine has speeded up, a centrifugal clutch engages and drives the rotor of the gas propulsion turbine. While the starting system disclosed in British Patent 943,449 relies upon a clutch to couple starting torque to the gas turbine after the starting motor has reached an initial speed, the disclosed system does not address energy consumption during starting.
Conventional electric starting systems for gas turbines directly couple the gas turbine to the electric starter motor. This configuration has the disadvantage that loading of the drive shaft of the electric motor with the rotor of the gas turbine causes considerable starting current to be drawn when the starting sequence for the gas turbine is initiated. The drawing of high starting currents degrades the battery voltage which can interfere with specifications of the battery for applications in airframes, such as that proposed for the Boeing 737-500 in which the starting motor may not draw more than 370 amps or degrade the battery voltage to less than 18 volts. Additionally, electric motors reach peak efficiency at approximately 50% of their maximum speed with minimum efficiency occurring at starting. As a result, starting losses are minimized by accelerating the electric starting motor as rapidly as possible up to the speed at which the turbine is self-sustaining.